Ion-Molecule Reactions in a Nitrogen-Benzene Plasma: Implications for the Destruction of Aromatic Compounds

Transcription

1 Ion-Molecule eactions in a Nitrogen-Benzene Plasma: Implications for the Destruction of Aromatic Compounds S. Williams, S. Arnold, A. Viggiano, and. Morris Air Force esearch Laboratory / Space Vehicles Directorate / VSBP ajesh Dorai and Mark J. Kushner / Optical and Gas Discharge Physics 52nd Annual Gaseous Electronics Conference Norfolk, VA - 6 October 1999

2 Introduction Dielectric barrier discharges (DBDs) are attractive plasma sources for remediation of toxic gases, such as N x O y, SO 2, and VOCs, due to their ability to operate at higher pressures with moderate applied voltage. However, the destruction of aromatic compounds in these low temperature plasmas is problematic due to the small rate of ring-cleaving reactions by plasma generated oxidizing radicals at ambient gas temperatures. We report on a combination of laboratory flow tube kinetics measurements and computational modeling to investigate the question of whether a nitrogen plasma can be used for the destruction of waste aromatic compounds based on ion-molecule reactions.

3 Flow Tube Kinetics Ion-molecule reaction X products Buffer Flow v ate = -d[x ]/dt = k[x ][] is present in a large excess Pseudo first order kinetics k = 1 [] τ ln [X o ][X ] The reaction time is τ = L/v ion Plot ln[x ] vs [] and the slope = -kτ ln[x ] X X X L [] (x10 11 cm -3 )

4 Selected Ion Flow Tube (SIFT)

5 High Temp Flowing Afterglow Ceramic tube-1800 K Quartz tube-1400 K

6 C 6 Ion-Molecule eactions eactant Ion eco mbinat ion Ene rgy (ev) Temperature ange NO O N O Kr ( 2 P 3/2 ) N Kr ( 2 P 1/2 ) N Ar F Ne

7 X C 6 fi Products X = N 4 N N 2 Ar Ne E (ev) = Ion Products (300K) C C 6 H C 6 H C 5 H C 5 H l-c 4 H c-c 4 H C 4 H C 4 H c-c 3 H C 2 H C 2 H ate (molecule-cm 3 -s -1 ) 1.2 x x x x x 10-9

8 Benzene Breakdown Diagram Total Energy = E Ion E Int E Trans elative Abundance (%) N F Total Energy (ev) C C C C H n H n H n H n

9 Kinetics Summary Series of benzene reactions all proceed at collisional rate. Mechanism changes from association to non-dissociative and then dissociative charge transfer with increasing reactant ion energy. Primary and secondary dissociation product ions are observed. Isomeric form of C 3 H 3, C 4 H 4, and C 5 H 3 product ions was determined. Temp dependent branching fractions were converted to product ion breakdown curves. Pressure effect observed due to collisional stabiliztion of the charge transfer complex by He buffer. N reaction with benzene involves many non-charge transfer processes. Thanks to AFOS

10 DBD Model Description The basis of the model is to integrate the nonlinear ordinary differential equations describing the reaction chemistry over the residence time with the simultaneous solution of the equations for the circuit parameters. The rate coefficients for the electron impact reactions are obtained from a lookup table produced by an offline Boltzmann solver. Offline Boltzmann Solver Lookup Table of k vs. T e dn(t) dt Circuit Module LSODE ODE SOLVE N(t t), V, I

11 Dominating eactions c-c3h3 C2H2 HCN Ar C6H6 C6H6 N Ar C6H6 N Ar N2 C6H6 C 6 C 6 N4 N2 N2 C6H6 C6H5 H I-C4H4 C2H2 N2 N2 c-c3h3 C3H3 Ne C4H3 C2H3 C4H2 C2H4 Ne C6H6 Ne

12 Dissociative ecombination eaction ate (molecule-cm 3 -s -1 ) E C 6 = C 6 H 5 H 5.00 x 10-7 E C 6 = products 5.00 x 10-7 E C 6 H 5 = C 6 H 4 H 2.76 x 10-7 E C 6 H 5 = products 8.27 x 10-7 EC 6 H 4 = products 11.0 x 10-7 EC 5 H 4 = products 9.00 x 10-7 EC 5 H 3 = products 9.00 x 10-7 E l-c 4 H 4 = products 5.77 x 10-7 E c-c 4 H 4 = products 5.77 x 10-7 E C 4 H 3 = products 6.20 x 10-7 E C 4 H 2 = products 5.77 x 10-7 E c-c 3 H 3 = products 7.00 x 10-7 E C 2 H 3 = products 4.50 x 10-7 E C 2 H 2 = products 2.70 x 10-7 Thermal rate coefficients are listed. The model assumes a T -0.5 temperature dependence. The category "products" signifies species lacking a six membered aromatic ring ates in red are estimated. ates in black are measured (see references). eferences: 1) Lehfaoui et al., J. Chem. Phys. 106: (1997). 2) ebrion-owe et al., J. Chem. Phys. 108: (1998). 3) Mitchell and ebrion-owe, Int. ev. Phys. Chem. 16: (1997).

13 Experimental Setup Modeled A pulsed low pressure discharge has been modeled as follows: Dielectric discharge height = 2.5 mm. eactor pressure = 1 Torr Operating temperature = 300 K Single pulse input (ca. 100 ns duration) Input benzene = 0.1% Key assumptions are as follows: The dissociative recombination rates of many organic molecular ions have been measured, but the product distributions have not. It is assumed here that for C 6 e - and C 6 H 5 e - only only 50% and 25%, respectively, of the reactions produce products with the six membered ring intact. The remainder of the species are classified as products.

14 W-Value [ ev/molecule C 6 removed ] Air Force esearch Laboratory Effect of Energy on Benzene emoval (Ar/N 2 /C 6 ) Most of the benzene removal involves breaking of the aromatic ring since the reaction of Ar with C 6 predominantly produced l-c 4 H 4 and other lower order hydrocarbons % Ar 5.0 % N 2 0.1% C Concentration at 0.2 s [ ppm ] % Ar 5.0 % N 2 0.1% C 6 Benzene emoved Products Formed Energy Deposition [ J/L ] Energy Deposition [ J/L ]

15 Effect of N 2 on Benzene emoval (Ar/N 2 /C 6 ) W-Value [ ev/molecule C 6 removed ] End products mainly included the recombination products of l-c 4 H 4, C 6 H 5 c-c 3 H Concentration at 0.2 s [ ppm ] Benzene emoved Ar [ % ] Ar [ % ] Products Formed

16 Effect of Energy on Benzene emoval (Ne/N 2 /C 6 ) W-Value [ ev/molecue C 6 removed ] Most of the benzene removed resulted in breaking of the aromatic ring structure 94.9 % Ne 5 % N 2 0.1% C Energy Deposition [ J/L ] Energy Deposition [ J/L ] Concentration at 0.2 s [ ppm ] Benzene emoved Products Formed 94.9 % Ne 5 % N 2 0.1% C 6

17 Effect of N 2 on Benzene emoval (Ne/N 2 /C 6 ) W-Value [ ev/molecule C 6 removed ] Both benzene removal and energy efficiency improve with increasing Ne concentration Ne [ % ] Concentration at 0.2 s [ ppm ] Benzene emoved Ne [ % ] Products Formed

18 Modeling Summary The energy efficiency increases initially with increasing energy deposition and remains constant a higher input energies. Benzene removal increases with increasing input energy. The best results are obtained for pure Ar and Ne discharges which is due to the large rate of ring-cleaving reactions by plasma generated Ar and Ne ions. At lower input energies, Ne discharges are better degraders of benzene. The presence of N 2 inhibits the benzene degradation efficiency mainly because the energy efficiency in the absence of N 2 is much higher than in the presence of N 2. Dielectric barrier discharges operating with Ar or Ne show potential as sources for remediation of aromatic compounds. Thanks to NSF

19 N 2 C 6 Branching Fraction C H 6 6 C H 6 5 C H 6 4 C H 5 3 C H 4 4 C H 4 3 C H Temperature (K) C H 3 3

20 Parent Ion: C 6 elative Abundance (%) 100 all temp data K data only 60 photodissociation Total Energy (ev)

21 eaction Coordinate Diagram (c) (d) (e) (a) 87? (b) (a) C (b) C H H 6 5 H H (c) l-c H C H (d) c-c H C H (e) c-c H C H H fissure(s) H migration ing rearrangment and opening